Method for forming features upon a substrate

ABSTRACT

A material deposition process is disclosed in which apertures of a contact mask used therein have a constricted opening terminating in a `knife edge` in a sidewall thereof near the top mask side, especially within the top 25% of the mask thickness above the substrate. A process is disclosed in which the mask, in addition, has apertures which have larger dimension lower openings on a bottom side of the mask contacting the substrate than constricted openings near the top side of the mask. Single solder bump and &#34;bump on bump&#34; over BLM (ball limiting metallurgy) processes are disclosed which utilize such contact mask to reduce the damage and detaching of such features during processing and subsequent handling.

CROSS-REFERENCE TO RELATED PATENTS AND PATENT APPLICATION

This patent application is related to the following U.S. patent andpending U.S. patent applications, all of which are assigned to theAssignee of the instant application and the disclosures of which arehereby incorporated by reference: U.S. Pat. No. 5,634,268 to Dalal etal. entitled "Method for Making Direct Chip Attach Circuit Card"; Ser.No. 08/972,183 filed on even date herewith entitled "SelectiveDeposition Mask and Method for Making the Same" ; Ser. No. 08/476,474filed Jun. 7, 1995, entitled "Reflowed Solder Ball with Low MeltingPoint Metal Cap" now abandoned; Ser. No. 08/476,475 filed Jun. 7, 1995,entitled "Method for Forming Reflowed Solder Ball with Low Melting PointMetal Cap" now abandoned; Ser. No. 08/476,472 filed Jun. 7, 1995,entitled "Direct Chip Attach Circuit Card" now U.S. Pat. No. 5,796,591Ser. No. 08/740,571 filed Oct. 31, 1996, entitled "Flip Chip Attach onFlexible Circuit Carrier Using Chip with Metallic Cap on Solder" nowU.S. Pat. No. 5,729,896; and Ser. No. 08/846,930 filed Apr. 30, 1997,entitled "Capacitor with Multi-Level Interconnection Technology" nowU.S. Pat. No. 5,808,853.

FIELD OF THE INVENTION

The present invention relates to material deposition using a selectivedeposition mask, and more specifically to selective material depositionperformed in the fabrication of integrated circuits and wiring.

BACKGROUND OF THE INVENTION

Existing contact masks used for the selective deposition of metal padsin back-end-of-line (BEOL) integrated circuit manufacturing areapproaching the limit of their ability to define smaller features toincrease the density of integrated circuit (IC) chip arrays such asmemory or logic arrays. In moving to increased integration density,solder deposition and chip attach processes have had to be conducted atlower temperatures than before, in order to reduce the effects ofdifferential thermal expansion between the chip substrate and the maskor chip carrier during processing. In order to permit chip attachprocesses at lower temperatures, two step deposition processes have beendisclosed in the above-referenced U.S. patent application Ser. Nos.08/476,474 now abandoned and 08/740,571 now U.S. Pat. No. 5,729,591(Attorney Docket Nos. FI9-95-068 and FI9-95-164, respectively), in whicha low melting point (LMP) capping composition (e.g. tin or a eutecticcomposition of solder metals: tin and lead) is deposited to cover ahigher melting point solder feature. However, in practice, the cappingstep must be tightly controlled to form caps of sufficient volume overthe solder features while avoiding the deposition of the cappingmaterial at undesired locations on the substrate. As will be understood,the effects of misalignment due to differential thermal expansionbetween the substrate and the mask or between the first depositedfeatures and the mask must be better controlled.

"Knife edge" sidewalls, i.e. sidewalls having an edge which projectsinto the aperture opening, are formed in apertures of contact masks usedfor material deposition. The knife edge is a result of chemical etchingfrom both sides of a contact mask to form apertures therein.Conventionally, a knife edge comes to a sharp edge about halfway up theheight of the aperture sidewall. Such knife edge apertures areunderstood to have beneficial effects in maintaining a separationbetween the mask and deposited material, up to a height of depositedmaterial of 125% of the mask thickness above the substrate. However,when the height of the deposited material is greater than 125% of themask thickness, the deposited material can undesirably become detachedfrom the substrate upon removal of the mask. This problem is furtherexacerbated when the mask is used to define second features over firstraised features already deposited through the mask in a previousdeposition step, as described in Applicants' co-pending U.S. patentapplication (Ser. No. 08/476,474) now abandoned. In such case, evenslight misalignment of such conventional mask apertures to previouslydeposited features, whether thermal or mechanical in origin, can lead tothe deposited features being gouged or detached from the substrate whenthe mask is removed, regardless of the height of the features relativeto the mask thickness.

U.S. Pat. No. 5,359,928 to Blessington, deceased et al. ("the '928Patent") describes a paste screen printing stencil having apertureswhich are larger on a bottom substrate facing surface than on a toppaste receiving surface (FIG. 3, col. 2, lines 65-68). The '928 Patentnotes that a knife edge in the sidewall of apertures can lead todeposited solder paste being pulled away or smeared upon removal of thestencil (col. 6, lines 31-55). The '928 Patent proposes a solution tosuch paste screening problem which is very different from Applicants'invention, as will be described in the following. The '928 Patentdescribes using electroforming instead of etching to form screenapertures to eliminate the knife edge in the sidewall and to provideaperture sidewalls that are tapered and smooth. The '928 Patent furtherdescribes forming raised edges 13 around the bottom aperture openingsfor use in sealing the screen stencil against leakage of the liquidpaste around deposited features on the substrate.

As will be understood from the description of the invention to follow,the screen printing stencils described in the '928 Patent could not beused in the manner described herein for the invention in forming solderfeatures by vapor deposition onto semiconductor wafers. Electroformedpanels are known to have much less shear strength than etched panels ofrolled or forged metal. For this reason, electroformed panels deflectand can break too easily for use in a vapor deposition process asdescribed in the following. Secondly, the raised edges surrounding themask apertures described in the '928 Patent can place stress insemiconductor wafer substrates, causing indentation of polymer films andpotential cracking and breakage of inorganic dielectric films on suchsubstrates. Thus, it will be understood that the teachings of the '928Patent cannot be meaningfully applied to the problem solved by theinvention in depositing materials onto a wafer substrate.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for forming features upon a substrate which results in a loweredrate of undesired detaching of features.

It is another object of the invention to provide improved control indepositing cap features over closely-sized first features.

It is another object of the invention to provide a method of depositingfeatures which compensates for thermal expansion related misalignment.

Still another object of the invention is to provide a method ofdepositing larger volume cap structures over closely-sized firstfeatures.

Still another object of the invention is to provide a method of formingfused cap on solder structures having relatively large cap volumes.

These and other objects of the invention are provided by the mask of thepresent invention. Accordingly, a method is provided for depositingfeatures upon a substrate, which method includes contacting thesubstrate with a mask having apertures therein, each aperture having alower opening in a bottom side of the mask contacting the substrate anda constricted opening near a top side of the mask, and then exposing themasked substrate to a source of deposition material.

Preferably, a dimension of the lower opening of the mask is larger thana dimension of the constricted opening, more preferably between 10% andabout 35% larger than the constricted opening, and most preferably about25% larger than the constricted opening. Preferably, the sidewall ofeach aperture terminates in a knife edge at the constricted opening.

In a preferred embodiment, the constricted opening is located within 25%of a thickness of the mask from the top side of the mask. It is alsopreferred that the apertures be located in the mask according to theirrespective radial positions at an elevated temperature.

In a highly preferred embodiment of the invention, high melting point(HMP) solder features are capped by low melting point (LMP) materialfeatures upon a substrate, in a method including the steps of 1)contacting the substrate with a first mask having apertures therein,each aperture having a lower opening in a bottom mask side contactingthe substrate and a constricted opening near the top side of the mask,wherein a dimension of the lower opening is greater than a dimension ofthe constricted opening and the sidewall of each aperture terminates ina knife edge at the constricted opening, 2) exposing the maskedsubstrate to a HMP solder deposition source to form first depositedfeatures; 3) reflowing the first deposited features; 4) contacting thesubstrate with a second such mask; and 5) exposing the masked substrateto a LMP or eutectic material deposition source to form second depositedfeatures upon the first reflowed features. The second mask may differfrom the first mask in the placement and size of apertures therein.Apertures are placed in the first and second masks according to theirrespective predicted positions at temperatures reached during HMP andLMP deposition steps, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the placement and use of the mask of the present inventionin patterning material features on a wafer substrate.

FIG. 2. illustrates the sidewall shape of an aperture in a mask having aconventional knife edge sidewall.

FIG. 3 illustrates the sidewall shape of an aperture in a maskconstructed according to the present invention.

FIG. 4 illustrates the relationship between an aperture of the mask ofthe present invention and a material feature which is depositedtherethrough.

FIG. 5 illustrates a structure formed in a deposition process in which afirst feature is deposited.

FIG. 6 illustrates a structure formed in a deposition process in which afirst deposited feature is reflowed.

FIG. 7 illustrates a structure formed in a deposition process in which asecond feature is deposited over a reflowed first deposited feature.

FIG. 8 illustrates a capped structure formed in a deposition process inwhich a second feature deposited over a first feature is reflowed.

FIG. 9 illustrates a fused structure formed in a deposition process inwhich a second feature deposited over a first feature is fused into thefirst deposited feature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the placement and use of the mask of the present inventionin patterning material features on a substrate such as a semiconductorwafer. As shown in FIG. 1, mask 10 is held in close contact withsubstrate 12 by the force of springs 14 against the back side ofsubstrate 12, mask 10 being held in place by ring 16 in chuck 18. Asevident from the arrangement shown in FIG. 1, mask 10 must besubstantially rigid and not brittle, in order to avoid excessivedeflection or cracking (and resultant loss of contact with substrate 12)due to the compressive force of springs 14.

FIG. 2 is a diagram showing a cross-section of an aperture 20 within aconventional mask 11 used for depositing material, especially metal upona substrate 12. As shown in FIG. 2, aperture 20 is provided with a knifeedge 22 in the sidewall thereof about halfway up the height of theaperture thickness from the substrate. As used, knife edge 22 ofaperture 20 in conventional mask 11 permits features of trapezoidalcross-section to be deposited, when the deposited features have heightless than about 100% of the mask thickness above the substrate. However,when the deposited features have height greater than 100% of the maskthickness, an unacceptable proportion of features deposited through suchmasks have been observed to become damaged or detached from substrate 12upon removal of mask 10. This problem has been traced to two primarycauses: 1) the continuity that results between features of such heightdeposited through the mask apertures and the conformal film of metalwhich accumulates on the top surface of mask 12 during deposition; and2) gouging of the deposited features 24 by knife edge 22 in the sidewallof mask 10 due to thermally related shifts in the relative positions ofmask 10 and the deposited features. Gouging and loss of adhesion areobserved in still greater proportions when alignment is less wellcontrolled and materials are deposited layer upon layer in a multipledeposition process.

FIG. 3 shows a cross-sectional view of an aperture 32 of a mask 30constructed according to the present invention. As shown in FIG. 3,aperture 32 contains a constricted opening 34 in sidewall 36 thereof.The constricted opening 34 terminates in a knife edge 38 near the top 40of the mask aperture (on the top side of mask 30, i.e., the side whichis nearest the source of the deposition material).

As used herein, the term "knife edge" is meant a fine edge at theintersection of sidewall surfaces. However, it will be understood thatthe term "knife edge" also denotes such an edge which is dulledunintentionally or intentionally during manufacture or as a result ofhandling or use.

Preferably, the knife edge 38 is situated in the sidewall 36 within 30%of the mask thickness from the top surface of mask 30, and morepreferably between 10% and 30% of the mask thickness down from the topsurface 40 of mask 30.

Preferred dimensions of aperture 32 which have been found to workespecially well in a deposition process of solder and solder with tincap are as follows. For deposition of solder pads of diameter nominally4.0 mil (±0.3 mil), a mask 30 of thickness 4.0±0.2 mil is selected andapertures are formed in mask 30 having diameter at knife edge 38 of4.0±0.3 mil. Knife edge 38 is formed less than or about 1.2 mil belowthe top surface 40 of mask 30, resulting in an aperture which has aminimum diameter of 4.0±0.3 mil at the constricted opening 34, i.e. atknife edge 38, and a maximum diameter at the lower opening 39 in thebottom surface 42 of mask 30 of 5.8±0.3 mil.

While the foregoing relative and fixed dimensions are preferred, thesingle overriding constraint is that the lower aperture opening 39 inthe bottom surface 42 of mask 30 be at least 10% larger than theconstricted opening 34.

The locations of apertures in mask 30 are thermally compensatedaccording to their predicted positions at the deposition processtemperature. The positions, in turn, are predicted according to therelative coefficients of thermal expansion of mask 30 and substrate 12and the distance of apertures with respect to one or more fixedreference points on the mask 30 or substrate 12.

A preferred method of fabricating mask 30 is as follows. A sheet of coldrolled molybdenum is covered on both top and bottom surfaces 40 and 42with a photoresist material (not shown) and then patterned byphotolithography to form opened areas in the resist over the apertures32 to be formed. Opened areas of the resist pattern over mask 30 arethen etched by processes which differ in etch rate for top and bottomsurfaces 40, 42 to form apertures 32. One method for performing suchdifferential etching is by pulsing an etchant composition from a spraynozzle towards the top and bottom surfaces of mask 30 at differentrates, such that the time of contact of etchant to the bottom surface 42is greater than the time of contact to the top surface 40, preferably bya factor of four. Preferably, etching of apertures 32 is conducted fromboth top and bottom surfaces 40, 42 simultaneously in order to reducefabrication time, although this is not a requirement for producingapertures of the required cross-section.

Alternatively, instead of varying the rate of pulsation in etchanttowards top and bottom surfaces 40, 42 of mask 30, the volume of etchantprovided in each pulse to top and bottom surfaces 40, 42 can be variedsuch that proportionately less etchant is provided to top surface 40than bottom surface 42. In still another alternative method, the volumeof etchant and rate of pulsation for top and bottom surfaces 40, 42 canbe held constant, while the concentration of active reagents provided toeach surface 40, 42 is varied. Preferably, in this alternative method,the etching of top and bottom surfaces 40, 42 would be performed atdifferent times in order to best control the etching of each surface ofmask 30.

Mask 30 is utilized in deposition processes as follows. In a one stepdeposition process for depositing single features, for example, simplesolder bumps, mask 30 is thermally compensated to 75±5 degrees C. Aswill be understood, the difference in the coefficients of thermalexpansion between the metal mask 30 and the substrate 12 ofsemiconductor material cause the positions of apertures 32 to shift withtemperature relative to substrate 12. The thermal compensation of mask30 to a nominal deposition temperature of 75 deg. C. allows for shiftsin the position of each aperture 32 relative to the desired site ofdeposition when the substrate reaches the deposition temperature.

FIGS. 5-6 show the relationship of mask 30 to the feature 50 asdeposited (FIG.5) and after reflowing (FIG. 6). As will be understood,the geometry of the aperture 32 in mask 30 provides a greater spacingbetween the knife edge 38 and the deposited feature 50, as compared tothe spacing provided by a conventional mask in which the knife edge issituated near the midpoint of the mask thickness (FIG. 2). This greaterspacing in mask 30 permits the knife edge 38 to shift a relatively greatamount relative to the feature 50, thereby permitting mask 30 to be usedfor deposition of solder bumps on wafers having diameters up to 300 mm.(12 in.).

The inventors have found the mask 30 of the present invention to beespecially advantageous in a process of depositing "bump on bump"features over BLM (ball limiting metallurgy) pads. With reference toFIGS. 5 through 8, in one such exemplary process, a BLM pad 48 is formedby vapor deposition of Cr, Au, Cu, Ni or any combination thereof througha mask 30 onto substrate 12. For such BLM deposition, mask 30 isthermally compensated to 225±5 deg. C. The substrate reaches thatnominal temperature during deposition of BLM pads which precedes thedeposition of solder bumps.

Without removing the 225 deg. C. compensated mask 30, first solder bumpsof HMP composition are deposited at a deposition temperature of about 60deg. C. The resulting structure of the deposited first solder bump 50over the BLM pad 48 is shown in FIG. 5. Here, the displacement of thetrapezoidal solder bumps 50 relative to the BLM pads 48 is not a concernbecause surface tension of the solder bump when molten during asubsequent reflow operation brings the solder bump into alignment withthe BLM pad 48.

Mask 30 is then removed and solder bumps 50 are reflowed by heating,which then results in the structure shown in FIG. 6. A second vapordeposition step is then performed in which caps of tin or of an eutecticcomposition 52 are vapor deposited over first solder bumps 50 throughapertures 32 of a second mask 30, thermally compensated to 75 deg. C.,resulting in a structure as shown in FIG. 7. Mask 30 is then removed toexpose solder "bump" features 50 having caps 52. After an optionalreflowing step (FIG. 8), mask 30 is removed to expose reflowed tin caps54 which adhere to the top surface of the solder bumps. The uniquegeometry of the apertures 32 within mask 30 improves control over theformation of the caps 52, or 54.

With reference to FIG. 9, when the cap is of an eutectic composition,the reflowing step results in a fusion of the eutectic cap 52 with theunderlying solder bump 50. Such fusion provides strong mechanicaladhesion of the fused eutectic cap 52 to the underlying solder bump 50which lowers the incidence of undesired detaching of caps from solderbumps during transporting, chip attach processing and electricaltesting, as caused by shock, vibration and friction.

The placement of knife edge 22 near the top of mask aperture sidewalls30 results in several important benefits. In cases where it is desiredto prevent the interaction of the deposited cap 52 with structuresunderlying the solder bumps, e.g. BLMS, because of possible reactionsbetween BLM metals (usually of Au, Cu or Ni composition) and tin, knifeedge 38 placed in the upper sidewall of aperture 32 inhibits the flow ofliquid tin, or the liquid eutectic composition, downward along thesurface of solder bump 50. An equally important benefit of the placementof knife edge 38 in the upper sidewall is realized in that a greaterquantity of tin 52 can be deposited on the surface of solder bump 50than is possible with existing contact masks (FIG. 2), thereby resultingin larger tin cap structures 52. In addition to the foregoing, asdescribed above in reference to single feature or "bump" deposition, theplacement of knife edge 38 near the top of apertures 32, and the conicalshape of apertures 32, which grow larger with increasing proximity tothe surface of substrate 12, result in a lowered incidence of undesiredgouging or detaching of features when mask 30 is removed from substrate12.

Utilization of the instant process results in larger volumes for eachLMP cap or eutectic cap. This, in turn, permits larger chips (ICs) to bejoined to packaging substrates, because the larger LMP or eutecticcomposition volume provides larger collapsibility and compensates forexpected greater variations in the planarity of semiconductor chip orpackaging substrates.

While the invention has been described with reference to certainpreferred embodiments thereof, those skilled in the art will understandthe many modifications and enhancements that can be made withoutdeparting from the true scope and spirit of the invention as set forthin the claims appended below.

What is claimed is:
 1. A method for depositing features upon asubstrate, comprising the steps of:contacting said substrate with a maskhaving apertures therein, each said aperture having a lower opening in abottom mask side contacting said substrate and a constricted openinghaving a knife edge located from within 10% to 30% of a thickness ofsaid mask from a top mask side, said lower openings having a diameter atleast 10% larger than a diameter of said constricted opening; depositinga composition onto said masked substrate of said knife; and removingsaid mask from said substrate.
 2. The method of claim 1 wherein theposition of each said aperture within said mask is thermally compensatedto a temperature said substrate reaches during said depositing.
 3. Amethod for forming capped features upon a substrate, comprising thesteps of:contacting said substrate with a first mask having aperturestherein, each said aperture having a lower opening in a bottom mask sidecontacting said substrate, and a constricted opening terminating in aknife edge in a sidewall of said aperture from within 10% to 30% of athickness of said mask from a top mask side, a dimension of said loweropening being at least about 10% greater than a dimension of saidconstricted opening, the position of each said aperture in said firstmask being thermally compensated to a first temperature; depositing afirst composition onto said masked substrate at said first temperatureto a height above said knife edge to form first deposited features;removing said first mask; contacting said substrate with a second maskhaving apertures therein, each said aperture having a lower opening in abottom mask side contacting said substrate and a constricted openingterminating in a knife edge from within 10% to 30% of a thickness ofsaid mask from a top mask side, a dimension of said lower opening beingat least about 10% greater than a dimension of said constricted opening,the position of each said aperture in said second mask being thermallycompensated to a second temperature different from said firsttemperature; depositing a second composition having lower melting pointthan said first composition onto said masked substrate at said secondtemperature to a height above said knife edge to form second depositedfeatures upon said first deposited features; and removing said mask. 4.A method for forming high melting point (HMP) solder bumps capped by lowmelting point (LMP) material features upon a substrate,comprising:contacting said substrate with a first mask having aperturestherein, each said aperture having a lower opening in a bottom mask sidecontacting said substrate and a constricted opening terminating in aknife edge in a sidewall of said aperture from within 10% to 30% of thethickness of said mask from a top mask side, a dimension of said loweropening being greater than a dimension of said constricted opening, theposition of each said aperture in said first mask being thermallycompensated to a first deposition temperature; depositing onto saidmasked substrate, sequentially, a ball limiting metallurgy at said firstdeposition temperature and a high melting point solder deposition sourceto form first deposited features having height extending above saidknife edges in said apertures; reflowing said first deposited featuresto form solder bumps; removing said first mask from said substrate;contacting said substrate with a second mask having apertures therein,each said aperture having a lower opening in a bottom mask sidecontacting said substrate and a constricted opening terminating in aknife edge in a sidewall of said aperture from within 10% to 30% of thethickness of said mask from a top mask side, a dimension of said loweropening being greater than a dimension of said constricted opening, theposition of each said aperture in said second mask being thermallycompensated to a second deposition temperature lower than said firstdeposition temperature; and depositing onto said substrate a low meltingpoint (LMP) composition to a height greater than said knife edges insaid apertures of said second mask to form second deposited featuresupon said first deposited features.
 5. The method according to claim 3wherein said second composition is a eutectic composition.
 6. The methodaccording to claim 5 further comprising the step of reflowing saidsecond deposited features before removing said second mask.
 7. Themethod of claim 4 wherein said LMP composition is a eutecticcomposition.
 8. The method of claim 4 further comprising the step ofreflowing said second deposited features before removing said secondmask.